Today's Message Index:
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1. 07:33 AM - Re: Interesting thread on overvoltage event (Jan de Jong)
2. 01:43 PM - Re: Is a battery contactor really needed? (Robert L. Nuckolls, III)
3. 09:29 PM - Re: Is a battery contactor really needed? (rv8ch)
Message 1
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| Subject: | Re: Interesting thread on overvoltage event |
Rotax, LiFePO4 and OVP
LiFePO4 batteries do not have a voltage limiting effect in OV situations
as lead acid batteries do.
In Rotax systems with builtin 20A PM alternator the capacitor on the
output of the R/R can give a little time for OVP to operate.
Some ruminations.
With the standard 22mF, 25V capacitor there is 9V room above 16V.
DO160-capable devices can take 40V for 100ms, 24V above 16V.
Say the alternator is capable of an excess 14A or 14,000mC/s.
The bus voltage will climb at 14,000/22 = 636V/s = 0.64V/ms.
The 9V room to 25V is consumed in 9/0.64 = 14ms.
With a 22mF, 40V capacitor there would be 24/0.64 = 37.5ms available
until the DO160 limit is reached.
If a 47mF/40V capacitor were installed the bus voltage would climb at
0.30V/ms and it would take 24/0.30 = 80ms for the DO160 limit to be reached.
At 1500 RPM (idle) the ripple frequency is 10 x 1500 / 60 = 250Hz (there
are 10 coils I believe), the ripple period is 4ms (each of these
consists of 2 ripples, not quite identical (different rectifiers)).
Shorter periods at higher RPM.
A fast OVP would sample several ripple periods before deciding on an OV
condition.
Relay release time (10ms or so (diode/zener question)) would be added.
In total 20 or 30ms may be enough.
All in all limiting the potential voltage excursion with LiFePO4 using
fast OVP and a somewhat oversized R/R capacitor seems doable for the
small engine users among us.
Don't know who supply fast OVP though.
Don't know if nuisance tripping can be avoided.
Jan de Jong
Message 2
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| Subject: | Re: Is a battery contactor really needed? |
At 03:35 PM 4/12/2017, you wrote:
>
>There seem to be a lot of advantages to not disconnecting the
>alternator from the battery, so I'm wondering if the battery
>contactor is really necessary?
>
>The things I can see as advantages are that it provides an extra
>layer of protection from accidental starter engagement, and provides
>a big switch that can be used to disconnect the battery from everything.
>
>I'm curious if there are other advantages that I'm not seeing.
There is a legacy train of thought that goes
back nearly 100 years . . . when on short final
to the rocks, it's a good idea to suppress
risk to the smallest values.
Hence, the POH for most if not all TC aircraft
calls for shutting off the fuel and killing
the electrical system. Getting the electrical
system stone-cold calls for independent, positive
control over power sources like alternators, generators
and . . . you guessed it . . . batteries.
Of course, the contactor is also a hedge against
a stuck starter contactor . . .
Don't know what 'advantages' might be secured
with permanent pathways between alternators
and batteries.
Bob . . .
Message 3
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| Subject: | Re: Is a battery contactor really needed? |
nuckolls.bob(at)aeroelect wrote:
> At 03:35 PM 4/12/2017, you wrote:
> Don't know what 'advantages' might be secured
> with permanent pathways between alternators
> and batteries.
Thanks for the feedback gents, looks like the off field landing scenario slipped
my mind when thinking about this. The "advantages" I was thinking of were one
less component, and the possible avoidance of a load dump if the alternator
is pumping hard and the battery is disconnected. Avoiding issues with a load
dump are pretty easy by following the recommended designs in the 'connection.
Thanks for the tips!
--------
Mickey Coggins
http://www.rv8.ch/
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=468379#468379
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